Group III nitride semiconductors have a direct transition band structure-and exhibit bandgap energies corresponding to the energy of visible to ultraviolet light. By virtue of these characteristics, Group III nitride semiconductors are employed at present for producing light-emitting devices, including blue LEDs, blue-green LEDs, ultraviolet LEDs, and white LEDs (which contain a fluorescent substance in combination with such a nitride semiconductor).
Growing only a nitride-single crystal itself has been considered difficult, for the following reasons. Nitrogen, which is a constituent of the single crystal, has high dissociation pressure and therefore fails to be retained in the single crystal in, for example, the pulling method.
Therefore, a Group III nitride semiconductor is generally produced by means of metal organic chemical vapor deposition (MOCVD). In this technique, a single-crystal substrate is placed on a heatable jig provided in a reaction space, and raw material gases are fed onto the surface of the substrate, to thereby grow, on the substrate, an epitaxial film of nitride semiconductor single crystal. The single-crystal substrate is formed of, for example, sapphire or silicon carbide (SiC). However, even when a nitride semiconductor single crystal is grown directly on such a single-crystal substrate, large amounts of crystal defects, which are attributed to crystal lattice mismatch between the crystalline substrate and the single crystal, are generated in the resultant nitride semiconductor single crystal film; i.e., the epitaxial film fails to exhibit good crystallinity. In view of the foregoing, there have been proposed several methods for growing, between a substrate and a nitride semiconductor single crystal epitaxial film, a layer having a function for suppressing generation of crystal defects (i.e., a layer corresponding to a buffer layer), so as to attain good crystallinity of the epitaxial film.
In one typical method, an organometallic raw material and a nitrogen source are simultaneously fed onto a substrate at a temperature of 400 to 600° C., to thereby form a low-temperature buffer layer; the thus-formed buffer layer is subjected to thermal treatment (i.e., crystallization) at an increased temperature; and a target Group III nitride semiconductor single crystal is epitaxially grown on the resultant buffer layer (see Japanese Patent Application Laid-Open (kokai) No. 2-229476). Also, there has been proposed a method including a first step of depositing fine Group III metal particles onto the surface of a substrate; a second step of nitridizing the fine particles in an atmosphere containing a nitrogen source; and a third step of growing a target Group III nitride semiconductor single crystal on the thus-nitridized fine particles (see International Publication WO 02/17369 Pamphlet).
Such a method can produce a Group III nitride semiconductor single crystal exhibiting somewhat good crystallinity. However, with an aim to further improve in the performance of a semiconductor device, demand still exists for a Group III nitride semiconductor crystal exhibiting further enhanced crystallinity.
Important factors for evaluating the performance of a semiconductor light-emitting device are, for example, emission wavelength, emission intensity and forward voltage under application of rated current, and reliability of the device. A key indicator for determining such a reliability is whether or not current flows under application of reverse voltage (not forward voltage) to the device; i.e., the magnitude of the threshold voltage at which reverse current begins to flow. Such threshold voltage is called “reverse withstand voltage.” In recent years, demand has arisen for a semiconductor light-emitting device exhibiting higher reverse withstand voltage and, accordingly, a further improvement in crystallinity is required.